A number of applications require the delivery of a liquid or other fluid from one height to another, significantly higher height. In some applications, one can use a positive displacement pump to urge fluid from the lower level to the higher level. So long as the pump has sufficient power to overcome the force of gravity and lift the fluid to the desired height, this is a very effective way to pump fluids to a higher level.
It is not always possible or convenient to provide a positive displacement pump at the lower end of the height to be traversed. In some situations, it may be simply inconvenient to place a pump at the bottom. For example, if the fluid delivery system is used to pump a fluid from the bottom of a deep tank up to the top of that tank, it may be difficult to gain access to the pump at the bottom of the tank for routine maintenance or repair.
In other circumstances, it may be impossible or highly impractical to try to place the pump at the lower end of the fluid travel. For example, when one attempts to pump water or other fluids from an underground geologic formation up to ground level, it is impractical to place a suitable pump down into the bore hole used to gain access to the underground formation. Instead, one will typically pump the fluid by drawing a vacuum at ground level and drawing the water or other fluid up through a fluid delivery conduit of some sort.
This can be very effective for materials having relatively low vapor pressures, such as crude oil. With materials having higher vapor pressures, though, it can be difficult to withdraw the material from particularly deep geologic formations because the material will tend to volatilize at the vacuum levels which would be necessary to draw the material up to ground level against the force of gravity.
For example, if one is attempting to pump water from an underground water table which is more than about 20 feet (about 6 meters) below the ground surface, one generally cannot use a vacuum pump. In order to overcome the "head" of the water, i.e., the weight of the column of water, over such a vertical distance, one would need to draw a rather substantial vacuum. However, the water will tend to boil at such a low pressure, filling the column with relatively low density water vapor. This can lead to a highly inefficient pumping operation if one can get any water out of the system at all.
The system can be even more problematic if the fluid delivery system is attempting to deliver a liquid which has a higher vapor pressure. For example, ground water can be contaminated with hydrocarbons having relatively high vapor pressures, e.g., gasoline or fuel oil. These contaminants will tend to form a layer of the lighter hydrocarbon material on top of the water table. One can try to remove this layer of hydrocarbon by pumping the top layer of the underground fluid up through a delivery conduit. If the hydrocarbon being extracted has a relatively high vapor pressure, though, this can make effective recovery rather difficult.